Safety control system for electrically operated heating means



Oct. 21, 1952 SAFETY CONTROL c. MAIN 2,614,621 SYSTEM FOR ELECTRICALLYOPERATED H EATING MEANS Filed Sept. 8, 1947 2 SHEETS-SHEET 1 w$ iv wx Yaw $5 w a QM N5 g Qfi mm. w

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Oct. 21, 1952 Filed Sept 8, 1947 R. C. MAIN SAFETY CONTROL SYSTEM FORELECTRICALLY OPERATED HEATING MEANS 2 SHEETS-SHEET 2 &

Allll Ebb e126 H15 42 mm? 6. Main.

INVENTOR.

Patented OctQZl, 1952 2,614,621, S'IPATENT OFFICE ,{SAFETY CONTROLSYSTEM FOR ELECTRI NCALLY OPERATED HEATING MEANS Robert C. Main, .LosAngeles, -Calif.,' assignor. to Robertshaw-Fulton Controls Company, acorj poration of Delaware Application September 8, 1947, Serial No.7712,754

This invention relates to control systems for electrically: operateddevices such as solenoid fuel valves; embodied in heating: apparatus.and,

more-5 particularly, to combined-l temperature,

safetyand ignition systems ofthe electrical type for use inconnectionwith gaseous fuel burners. --.A---system-- having the indicated featuresmay conveniently be arranged with a single solenoid valvesfor bothtem'perature and safety "control forfthefuel burner. I ergized: from asource of current supply at line voltage: Inwsuch installations,however, the

control device suchsas the room temperature thermostat in space heating"applications, is not advantageously arranged if. it is alsorequired tooperate at line voltage. Special transformers and low' voltage solenoidvalveshave been used in -conjunction with such temperature controldevices, in order to avoid this difliculty. Such. added equipmentdetracts from the desired simplicity of the basiccircuit and necessarilyinvolves additional expense in initial cost and upkeep.

An-obiect ofithis invention-is topcontrol a fuelburnin apparatussafely-and in accordance Other .objects and advantages willbecomeapparentfrom the following description taken in connection withthe. accompanying drawings,

wherein; j

Fig. l is a schematic view of the improved control-systemfor controllingthe flow of fuel ttpa fuel burning apparatus safely and'in accordancewith desired temperature conditions; and- Y 'l' ig-. .Zisa similarschematic .view of a modification of the improved control system.

Referring more particularly toFig. 1 of the drawings, the main burner Iis shown as being ,suppliedwith fuelzfrom a-main fuel pipe [2under-control-of themain fuel Valve cock- I4 whiclrincludes adouble-break main switch- 5 adapted to-be operatedto closed-positionwhen the cock M isxopened and tobe normally opened when the cook 14 isclosed. A pilot burner 18 is located in proximity of the main burner Illto ignite'the fuel flowing therefromandis'sup The solenoid valve is en-.

13. Cl'aims. "(curse-[124 plied with. fuel. by a. o-f'iv the .main. fuelcock means 2-2 preferably inthe form of a coil offre-j 'sistance' wireis located in lighting proximity to the-pilot .burner I8 to ignitethefuel issuing a controlling the-flow of fuel to the main'burnertakeqtheform ofLanf electromage l0. and may [4. Electric ignitingtherefrom a V Electrically operable means is providedfor netic. valve 24normallybiased'to closed position by gravity, spring means orthelikev to'-the..,supply,of,fuelto-themain burner I01 The MY only. The valve 214forinsthe-singlecontrol "scribed hereinafter.

valve. 24 .has.1a:.solenoid energizing coil 26 oper ati'velyl associated.therewith for causing it" to .openf..when .ie'nergyf. of plied to. thecoil 26. .lls will' be apparent, the supply, ofufuel to the. main burnercontrol of both themain fuel'cock l4 and'the vnvez while thesupplyoflfuel to the pilot burner; l 8 is under control of the main.fuel cock deviceof this inventionfor providing both safetyandteinperature control .of the main burner I0. In order to secure thisdesiredidual form of control .an electronic control ci'ro'uitiisutilized. An electron discharge device 28of the thyratron type is{provided for controlling operation of .the valve 24 in conjunction withelectron device 28 isindirectlyheated by a heating. filament together,with a control and anode 38 within the usual gas-filled envelope. 40.,As is customary in such.devices, the

control grid 34 is located in proximity of the cathode 230 while theshield gridi36is located "between the control .grid'34 andthe anode .38.

system and comprises 40.

Astep-down transformer 42 is included infthe Mand a. secondary 4B.nected at one end through one-pole ofjthe switch. H5v to a line wire 48.The bppositeend of the primary is connected to one terminal positeterminal of this pole of switch It is connected-to a line wire 52.While'any suitable voltage maybe employed depending on, the volt:

' .ageofthe power source available and; the op-"i eratingcharacteristicsof vtheelectron device'28 this instance is connected to .a..115-V01tsource.

of alternating current and. the secondary 46 is i and thevalve 24., theprimary winding 44in designed to operate at approximately sixvolts.Thethermal cutout 50 is provided with a heater 54- in the form: of acoil of wire 'of suitable resistance characteristics. .Thus, thecutjouti50 is capable. of opening the circuit of 'the 'p r' p p ssage ofovercurrent there;

throughdue'to its'series-connection in this circonduit 20 under controlprevent) a sufficient. value. is. sup- I0 is under 'means to be 'de- Thecathode 3% of the 32..and theseelements are housed,

grids, shield gridg36 a line voltagev primary 7 The primary .44 is con-,ofhthe opposite pole of switch l6] through the I terminals of a thermalcutout 5i). and .theopcuit. Moreover, the cutout Ell is also capable ofopening: the circuit of the primary 44 whenever the heater 5% becomessufiiciently heated by passage of current therethrough beyond apredetermined value.

iii

A heat responsive resistance. means is located in proximity of theheating means to 'be heated 1 by radiation therefrom. Specifically,theheat responsive means takes the form of a resistor 56 having normallya relatively low resistance and a positive temperature coefficient ofresistance. The resistor 56 is exposed'to the flame of .the pilot burnerI8 and is adapted to vary in resistance value when heated thereby. Theresistor 56 forms one of a pair of resistors,v the other of whichv isdesignated 58 and need have no specialthermal resistancecharacteristics. An addi tional resistance means is arrangedin the formof two resistor sections 66, 62 defined by a tap' 54. As willhereinafterbe apparent, the two pairs of resistors 55, 58 and 6B, 62each :"orm potential dividers across the voltage sourcefor diiferentpurposes connected with thecon'trol of the operation of the electrondischarge device 23.

Further meansfor controlling the operation o'f'the electron dischargedevice'28 include, in the embodimentshownin Fig. l, a thermallyresponsive means or thermostatic switching means in the form of aconventional room thermostat 66 which is located in a space to be heated,by .the heating means and is thus responsive to tem-,

peratures ambient thereto. The thermostat 65 comprises the usualbimetallic coil or arm .58 carrying a movable contact for cooperationwith a relatively fixed contact I2. The bimetallic arm 58 is connectedthrough a resistor M to' the control. grid 'Another resistor I6 isincluded in the connection from the shieldgrid S3 to the resistors56,158. i

Time delay mechanism including a relay 78 and timing device 80 formspart of the control system. The relay I8 includes a coil 82 and a pairofspaced armature means 84, 8B operable The armature means 84, 86 aredethereby. signed to cooperate with a pair of fixed contacts 8 8, Gil,respectively, when the coil 82 is efiectively deenergized. Uponenergization of the coil 82,

the armature means 84 is designed to cooperate with a fixed contact 92While the armature means 86 merely ceases to cooperate with the contact98. The timing device 88 includes a bimetallic strip 9 having' a heater95 in the form of acoil of wire associated in heating proximitytherewith. "The bimetal strip 94 is adapted to Warp when heated by theheater 95 and cooperate with afixed contact 93. I The various elementsof the system thus described are completed by the provision of aresistor Iflil for-limiting the value of the current supplied to theigniting means 22. It will be apparent that the voltages and resistancevalues oi the systemwill be arranged to provide the combination mostfavorable to contact life and Y safety in Wiring. In the subsequentdescription of the operation of the system the various cir- 1 tricsupply to the primary M of the transformer 42 are turned onsimultaneously by operation of the main fuel valv'e l l. Thus, fuel willflow in the conduit 28 to the pilot burner I8 and the transformer lzwill simultaneously be energized. The current flows from the primary 44to the heater of the timing device 80 through the following circuit:upper terminal of primary 44. wire 502, armature 84, contact 83, wireIll l, coil 95, and wire Hi6 to the lower terminal of the primary 44-Meanwhile, the secondary 46 is energized and a portion of thetransformer output is utilized to energize the igniting means 22 asfollows: lower terminal of secondary 45, wire I03, armature 86, contact9!], Wire IIB, resistor Hill, igniting means 22 and wire I I2 back tothe upper terminal of transformer secondary 46.

Upon the igniting means 22 reaching igniting temperature the fuelissuing from the pilot burner 581s lighted thereby but, due to the valve2:1 being still closedfthere is no flame produced at the main burnerIll. However, the thermal resistor 56 is heated by the pilot burnerflame and cooperates with the'resisvor 58 to establish the bias for theshield gricl 3B of the electron discharge device 23. The resistors 58,58 are connected in series circuit across the secondary 46 in thefollowing circuit: lower terminal of secondary 45, wire H4, resistorWire'IIG, resistor 55 and Wire H2 back to the upper terminal or"transformer secondary fiizi. "The resistors 58 and 55 also have a commonconnection to the shield grid 36 by resistor It? and wire H8 connectedto wire I IS.

Upon expiration of a few seconds fromthe initial production of a flameat the pilot burner, the heater coil 38 heats thebimetal strip 94sufficiently to cause it to warp and engage the contact The relay coil82; is then energized through the following circuit: upper terminal ofprimary Ml, wire 52, wire 12%, bimetal 94, contact fidwire I22, coil 82,Wire I24 and Wire I06 back'to the lower terminal of the primary 44. Thearmature means 84, 56, now move to their attracted positions; armaturemeans 84 moves into engagement with contact 92 Whilearmature means 86becomes disengaged from contact 90, As the armature means 86 and'contact99 control the described circuit to the igniting means 22, the latternow becomes deenergized. Moreover, the circuit to the heater coil96 isalso cleenergized for a similar reason. Dueto engagement of the armaturemeans 84 with the contact 92 an additional circuit is established forthe relay coil 82 and a circuit isestablished for the anode 3B of theelectron discharge device 28.

The relay coil 82 now becomes energized through the following circuit:lower terminal of primary M, wire I06, wire 124, coil 82, Wire I 26,Wire I28, contact 92, armature means 84 and Wire 5632 to the upperterminal of primary 44. Thus, although the bimetal strip 4 cools andbecomesdisengaged from contact 98, the relay coil 82 remains energizedthrough the described holding circuit. It will be apparent that in theattracted position of the armature means 84 the relay coil 82 remainsenergized from the primary side of the transformer 22 at approximatelyvolts. voltage is also applied to the anode circuit of the electrondischarge device 28' as follows: upper terminal ofprimary 44, wire I 92,armature means 8 3, contact 92, wire H8, valve coil 25, wire 638, anodecathode 38, wire I32 and Wire Hi5 to the lower terminal of the primary44.

The cathode 30 is heated by the filament 3 2 which is connected to thesix-volt secondary 45 byr-a'wire I34 fromroneendof theh'eatinggfilament32 to wire ll2 and'b'y'a wire l'36from 'theopposite-endiof the filament32 to. thewire'lM." Thus. if the electron discharge :device *2 8- is'iconductive, the? valve coil 26 will be energized throughathe describedoutput circuit to open' the valve-Hand admit fuel-from the fuel pipe l2.to. the burner l'llto' be ignited by the pilot burner l8." However, thedetermination of whether the electron. discharge'device 28 willac'onductor not depends on the bias on the control grid .34 and; shield grid 36,as will now be-apparent; Y The-cathode is maintained at. a fixed" po-'tential established by the position ofi'the' tap 64 relative to ther'esistors60, 62, this tap" being connected to the'cathode"3ll by'wirei385 The resistors 60. 62 are connected across "the filament supplyvoltage of "transformer-"secondary '46 by: connecting the free end ofthe resistor '69 to wire I I2 and the-free end of I resistor '62'to wire--l I45 The bias voltage for the control grid '34 is providedby'connecting-this grid 34 through the resistor Hand wire I42 to thebimetal arm of thethermostat 66 which has itsstationary con tact 12connected by wire I40 to-wirei'liz." The thermostat 661s adapted toclose its contacts .10; 12 upon asufiicient increase in temperatureAs'long as thecontacts-y'm,

ambient thereto.

'IZremain closed a voltage is supplied'to the control grid- 34 which" isof opposite phase to the voltage-supplied to the anode38. The-magnitudeof "this "voltage is such that the negative bias on the control grid 34prevents the electron dis: charge device 28 from becoming conductive andtl 1e'va1ve 24 thereupon remains closed.

As soonas-the contacts"), 12 are opened due to the thermostat I.negativebias onythe control grid-34 is sufllciently reduced .topermitthe tube-to become ccnductive. It will be understood that'the-foregoingdescrip tion of the "junction of the control grid 34 is contingentupon." the thermalflresistor' 56 being heated by the pilot burner'aspreviously described or,'as will now be explained; no conduction "bytheelectron discharge device '28" is possibl'ei for any jexp'ected'value ofbias voltage on the control jgrid34; v f Assuming that thethermostattdhas itscontacts ;.12'open,then the determinationwhether theelectron discharge depends upon the heated or unheated condition of thethermal resistor 56. When this resistor '56 is sufficiently heated bythe pilot burner l8; then the bias voltageon the shield grid as issues--ciently reduced 'to'permit the electron discharge device 28 to becomeconductive. Consequently,

open'or closed; When these "contacts'are closed;

thevoltage-applied to the control 'gri'd 34 is suflici'entlyout-*of-phase relative to" that applied to the anode '30 that the-device281s rendered nonc0nd-uct-ive.--

resistor 56 must be'sufficiently heated and (the thermostat contacts"-10; 12 must-be open. The

eflect' of either grid bias on the? conductivity-of 66 calling'for-heat'then the v device 28" isxconductive Thus, there; aretwocondit'ions to be-fulfilled" before the electrondischarge -'de'vi'ce 28 conducts currentto the solenoid-valve 24; The

thecutput circuit lof the electron discharge device .28suppliessuflicient energyto the coil'26 of the valve 24 to cause the device 28".is dependent upon, the; 'othergrid bias sow that the twor grid elements;:34, 36 are effectively: interlocked. 1 r

The :energy so *supplied, to the coil 260i valve 24 is half-wave orpulsating direct-i-current due tozitherectifying effect of, theelectronic device 28. Inthe event that. the flame of the pilot burnerIt! becomes extinguished,.then the potentialdrop acrosscthe' thermalresistor 56' is decreased as soon as '=this :resistor' starts. tocool... Therate of decrease ofthis" potential. could beaugmentedbyutilizing I a thermal :resistance element-subject to the heating effectof'current fiow'therethrough inplace .of the resistor58. The biasrofttheshield supply to the main. burner I 0; Since the: main switch I 6 a mustbe opened in order to deenergize the circuit of"therelay"coil 82 andsreturn'rthe armature means 84, 86 to theirvinitial positionfor-ignition of the pilot burnerIB, the system is not automaticallyrecycling but requires manipe ulation of themain fuel cook. 14' tobeginwasznew cycle-ofoperation.

' During-normal running operation of "the fsystem thepilot burner '18continues to produce "a flame; Control'of'the-main burner lll'throughth'e valve 24 is under control of the-thermostat 66. Asexplained in thecase of thethermal resistor-56 and shield grid 36;"the"'contacts lfl,

12 of the thermostat 56 are utilized to "vary the bias-of the controlgrid 34 'so-as to cause th'e'output currentof the electron dischargedevice 28 to v'aryaccordingly'and control the 'operation of the valve24. Thus, when the ambient temperature to-which the bimetal'arm 68is'subject increases toa predetermined degree; then'the contacts 10,

12 close; Consequently, an out-of-phase voltage "obtained-from thesecondary 46 through the describedcircuitis applied to the control' grid34 and the electron'discharge device ZB-isrendered non-conductive; Thevalve 24 thereupon closes toshut-off fiow-of'fuel to the main burner'I'U until the thermostat again calls for heat where .upon the contacts10, 12 open and the device 28 becomes conductive.

While'the temperature of the thermal resistor 56 will vary-over-a smallrange with changes in the "rate of fuel flow to the pilot burner l8 1and due to changes in temperature conditions ambient to theiresistor 56;these changesresult in only'a slightxvariationin the voltage of'theshield grid .and it is apparent that serious fluctuations in theelectron discharge device 28 will not result. Generally; the electrondischarge device 28 is operated in a .mannerito make use ofthe two grids'36, 34 thereof 'for separate control purposes while the'cathode 30 ismaintained at a fixed potential. This manneroi operating the electrondischarge device 28 gives an extremely flexible typeor control and inthe embodiment shown and described herein permits the use of athermostat for control of ambient temperatures while retaining desirableignition and safety control features.

The embodiment disclosed in Fig. 2 of the thermal cut-out is notincluded in the embodiment" of Fig. 2; "Otherwise, theessential-elemerits or theasystem' are-identical .With.:l)h0Se'.-p1'.e-I

viously disclosed and further description"-'is deemed unnecessary.Referring now' 'more particularly to Fig. 2 of the drawings, the controlgrid 34 and its resistance element '14 have a commonconnection to a pairof resistors M4 and M6. Thefree end'of resistor I44 is connected to theWire: H 4 and the free end of resistor I46 is connected to thewire l I2'50 that the pair of resistorsconstitute another voltagedivisioninetwork across the filament voltage supply of secondary 48:The-resistor 14d isreferably formed from 'a'semi-conductive material,such as a metallic oxide," having a= negative temperature coefiicient-ofresistance and is responsive to temperatures ambient thereto forvaryingin resistance value withvv'ariations.in'its temperature and is located"iniia'space to be heated by the. main burner 13;" I Y I The resistorldtis in the form of a control rheostat of any known type and ismanually ad-- ju'stable over a range of resistivitysettingscorresponding to different temperature values at which it is desiredthat the temperature control will operate:- tflwill be apparent that theresistor M4 could equally'wellhave a positive coefficient of resistanceand that in; such event the relative positions of the resistors M4 and145 would be interchanged from that shown, inFig. 2 ofthe drawings. Inany event, the total resistance of the voltage division network thusformed is kept as highas possible to reduce any tendency towardself-heating from current new to a minimum.

In the operation of the system illustrated in Fig. 2 of the drawings,the bias onithe shield griddfi will be reduced as described inconnection withthe-Fig. l embodiment to such a value that the electrondischarge device 28 will be rendered conductive only if the thermalresistor 56 is sufiiciently heated by the flame at the pilot burner 13.The bias of the control grid 34 will permit the electron dischargedevice 28 to conduct when the temperature as sensed by the ambienttemperature resistor; his is below the setting established by theresistor M6. Thus, the thermal resistonEB makes use of the shield grid36 to permit this feature to override any heat demand established by theambient temperature resistor I44.

It will be apparent that when the ambient temperature resistor Midisbelow the controltemperature, then the shield grid bias will permitconduction before the thermal resistor 56 is up to its maximumtemperature and, conversely, that when the resistor M41 is at or nearthe control temperature a flame failure'will result in a rapid :shutoiftime for the main burner Hi. It will further be apparent that, as in thecaseiof the Fig. 1 embodiment, no conduction by the electron dischargedevice 28 is possible for any value of resistivity of the resistor M4wheneverthe thermal resistor 56 is unheated by the flame at the pilotburner 8. On the other hand, when the thermal resistor 56 is heated bythe flame at the pilot burner it, then a condition exists wherebychanges in the ambient temperature as detected by the resistor his caneither permit the electron discharge, device 23 to conductor not, asrequired.

It will be apparent that in both embodiments of the invention theambient temperature control devicemakes use or extremely low voltagesand has a very high resistance either inherently as in the case of theresistor IN or by incorporation of the resistor is, In the case of thecontrol thermostat 56, thiscombination is most favorable to contactlife. Moreover, the combination of low voltage and high resistance alsoprovides the "ultimate safety in ,wiring arrangements. Space heatingapplications normally require a transformer and low voltage solenoidvalve in conjunction with the room-thermostat. In the present system;the cost of both the transformer and the solenoid; valve are, containedin the cost of the system as a whole. Since the two items represent asubstantial portion of the total sum,

' the expenseof obtaining electronic ignition and safety features ismaterially-reduced It will be apparent that for spaceheatingapplications the control thermostat 66 in the Fig. 1 embodiment or theresistor M4 as shown in Fig, 2 could be used as therocm thermostat or itcould-act as a'stack or high temperature limit control. For this lattertype of application a manual reset circuit breaker could be usedto tripupon occurrence of over-temperatures or when a flame failure-occurs.vThe device is also applicable to range ovensas well as space heatingdevices. In a rangeoven the rheostat Mt would act as the oven controland the resistor i 44 would be placed in the oven chamber;

. While a preferred embodiment of the invention and a modificationthereof have been shown and described it is apparentthat many othermodifications will occur to those skilled in the art. Hencethe inventionis not deemed to be limited to the forms shown and described orotherwise than as defined'by the scope of the claims appended hereto.- I

I claim:

1. In 'a temperature controlling system, a control device having a coiland armature means operable to a controlling position upon energizationof said coil, a timing device operable for controlling the energy supplyto said coil and having" a predetermined delay period during whichsaidcoil is eifectively deenergized, contact means cooperable with saidarmaturein. said controlling position thereof for establishing a holdingcircuit for said coil independent of said timing device,agrid'controlled electron discharge device having the output circuitthereof controlled by operation of said armature, means operable forproducing a temperature condition, electrically operable means connectedin said. output circuit and operable for controlling the last said meansfor establishing said temperature condition, and biasing means adaptedfor normally'applying a positive bias to said grid sufficient to rendersaid device conductive, said biasing means including anelementresponsive to temperatures ambient thereto and located to respond tosaid temperature condition for lowering the potential of said grid uponan increase in said temperature sulficiently to render said electrondischarge device effectively non-conductive.

2. The system claimed in claim 1 wherein said biasing means comprises aresistor and a thermostatic switchjin series circuit with said grid,said switch being responsive to temperatures ambient thereto for closingsaid series circuit upon an'increase in'saidtemperature and beingcooperable with said resistor for lowering the potential of said gridsufficiently to render said electron discharge device effectivelynon-conductive.

3. The systemclaimed in claim 1 wherein said biasing means comprises apair of resistance means, one said resistance means being responsive totemperatures ambient thereto and the other said resistance means beingadjustable for varying its resistance value, said pair of redependent ofchargedevice havingthe output' to 'render said electron 1yrion-conductive.

'egcregcai sistance means being cooperable for lowering the "upon.energizationof said coil, .a:timing device having contacts connected inseries circuit awith said. coilzandsaid source and having apredetermined delay period during which "said: coil is effectivelydeenergized, contact meanscooperable withesaidearmature in saidcontrolling position for establishing alholding circuitfor said coilinsaid timing device; an. electron .dis-

circuit thereof connected to said sourcefandincluding.said contactmeans, said armaturemeans and said-electrically :ope'rable means;and/biasing means for thet inputcircuit of said electron dischargedevice including an element responsive toztemperature's ambient theretocaused .by operation'of said heating' means for varying :said bias; inaccordance with variations in said temperatures sufficiently to'controlthe conductivity .of said electron dis:- e'device,

' 5.1A temperature.;c'ontrolling system;compris ing in combination;heatingimeans tolbe' con-- trolled, electrically: operableimeansvforc'onti-"olling said heating means, a source .of 1 electric energy,a: grid-controlled electron discharge device having; the output'cir'cuitzthereof connected to said 'source and -.including saidelectrically operable means,potential'divider'means connected to saidsource 'anditappedito maintain said electron .dis

icharge; device at a substantiallyifixed cathode potential,.andtaxipair-of' resistance means in pare alleloircuitwithsaidpotentialedivider means, one or said pair of resistance-meansbeingresponsive to. temperatures .ambient thereto caused by operation'io'f:said heating mea'n's;and: the ;other' of said .p'air .of: saidvresistance means: being adjustable for varying" itsrresistance' value;zsaidipair of; resistance means. being tappedrior, lowering .thepotentialrofnsaid' grid upon; an lincrease in :said temperature,sufiici'ently to" render: said; electron discharge 1 device effectively:non-iiconductive.

-.6.A' temperature controlling system; comprising in.combinationpheating; means; to be controlled zelectrically operablemeans for controlling. said-gzheating :means, a :source. of electricenergy, a: grid-controlled;electron discharge de .vicehavihgthe outputcircuit thereof connected to said. source-:and including saidelectricallyoperable:means;c,potentialdivider means connected to saidsource andtapped to maintain said electron discharge device at asubstantially fixed and continuously energized cathode potential, andthermostatic switching -means connected to said source and said grid andhaving contacts movable to closed position responsive-to tempera turescaused by. operation ofsaid. heating means for applying out-of-phasevoltage to said grid upon an increase in said temperature sufficientlydischarge device effective- '7 A temperatureand safety control systemfor a heatingmeans comprising electrically operable means forcontrolling said heating means, a source ,bfielectric energy; anelectron discharge device having anode and cathode elements andaplurality of grids, connections between said source, said anode; andcathode l n elements and said electrically operable :means, heatresponsive resistor means located in proximity of said heat- -a heatingmeans comprising electrically operable responsive resistor means beingconnected to said v ing means to be heated by radiation therefrom andbeing adapted to vary. in resistancewalue with variations intemperature, connectionsbetween said source, said resistor and one ofsaid grids :for varying the potential of saidone grid upon variations insaidztemperature sufiiciently to vary the conductivity :ofsaid electrondischarge device and cause operation ofs-aid-electricallyoperable means,and thermally responsivemeans located in a spaceto be heated bysaid-;:heating means andebeingconnected to.. said source-and another oneof said. grids, said thermally responsive means being. cooperable withsaid resistor for varying saidconductivity in accordancewith variationsin temperature. caused", by. operation of said heating means.

8.- A temperature and safety control system for means for controllingsaid heating means-1a source of electric energy, an electron dischargedevice having .anode' and. cathode elements'and a plurality of grids,connections betweenv said source, said anode and cathode elements andsaid electricallyoperable means, heat responsive resistor means locatedin-proximity of said heat* ing means to' be heated byradiation-therefrom and being adapted to vary in resistancevalue withvariations in temperature, connections between said source, saidresistorand oneot said grids for varying the potential of said one gridupon variations in said temperature sufiiciently to vary theconductivity of said electron discharge device and cause operation ofsaid electrically operable means, and second heat responsive resistormeans located relatively remote from said heatingmeans and being adaptedto vary in resistance value with variations in temperature, said secondheat source and another one of saidgrids and being cooperable with thefirst said resistor means for varying said conductivity in: accordancewith variations in temperature caused by operation of said heatingmeans.

' 9. A temperature and safety control system' for aheatingmeans-comprising electrically operable means for controlling saidheating means, a source of electric ener y, an electron discharge devicehaving anode and cathode elements and a plurality of grids, connectionsbetween said source,'said anode and cathode elements" and saidelectrically operable means, heat responsive resistor means located inproximity of said heating means to be heated by radiation therefrom andbeing adapted to vary in resistance value with variations intemperature, connections between said source, said resistor and one ofsaidgrids for varying the potential'of said one grid upon variations insaid. temperature sufliciently to vary the conductivity .of

said electron discharge device and cause operation 1 of saidelectrically operable; means, J and thermostatic switching means.connected tol'said source and another one of said grids andlibeinglocated "relatively 'remote from said heating means" but i responsive totemperature changes caused by operation i thereof, 1 said switchingmeans being operable for= varying the conductivity of said -elect'ron'-discharg e --jdevic' while said heat responsive resistor meansrernainste -'1... J Y j 1 v 10; A temperature and safety'control system for aheating means comprising electrically 11' operable means for controllingsaid heating means, a source of electric energy, an electron dischargedevice having anode and cathode elements and a plurality of grids,connections between said source, said anode and cathode elements andsaid electrically operable means, a first pair of resistors connectedacross said source and having a common connection with one of saidgrids, one of said first pair of resistors being variable in resistivitywith variations in temperature thereof and being located in proximity ofsaidheating means to be heated by radiation therefrom, a second pair ofresistors connected in parallel circuit with said first pair and havinga common connection with another one of said grids, one of said secondpair of resistors being variable in resistivity with variations intemperature thereof and being located relatively remote from saidheating means but subject to' temperature changes caused by operation ofsaid heating means, said first pair of resistors be ing adapted uponheating of said one resistor of said first pair for varying thepotential of said one grid sufiiciently to render said electrondischarge device effectively conductive, said second pair of resistorsbeing adapted upon heating of said one resistor of said second pair tovary the conductivity of said electron discharge device while said oneresistor of said first pair remains heated.

11. A temperature and safety control system for fuel burning apparatushaving main and pilot burners and a source of electric energy,comprising electromagnetic valve means for controlling the supply offuel to the main burner, acontrol relay having a coil and armature meansoperable thereby, a timing device having contacts controlling theenergization of said coil for a predetermined period during which saidcoil is effectively deenergized, means operable in one position of saidarmature means for establishing a holding circuit for said coilindependent of said timing device, an electrondischarge device havinganode and cathode elements and a plurality of grids, connections between said anode and cathode elements and said electromagnetic valvemeans controlled by operation of said armature means, heat responsiveresistor means located to be heated by a flame at said pilot burner forvarying in resistance value with variations in temperature and beingconnected across said source and to, one of said grids, a second heatresponsive resistor means located to respond to temperature conditionscaused by operation of saidmainburner for varying in resistance valuewith Variations in said conditions andbeing connected to another one ofsaid grids and across said source in parallel circuit with the firstsaid resistor means, and an electric igniter for said pilot burnerconnected across said source in parallel circuit with said first andsecond resistor means and controlled by operation of said armaturemeans.

12. A temperature and safety control system for fuel burning apparatushaving main and pilot burners and a source of electric energy, comprising electromagnetic valve means for controlling the supply of fuelto the main burner, a control relay having a coil and armature meansoperable thereby, a timing device havingcontacts controlling theenergization of said coil 'for a predetermined period during which saidcoil is efiectively deenergized, means operable in one position of saidarmature means for cseration of said armature means, heat responsiveresistor means located to be heated by'a flame at said pilot burner forvarying in resistance value with variations in temperature and beingconnected across said source and to one of said grids, thermostaticswitching. means connected to said source and another one of said gridsand being located to respond to temperature conditions caused byoperation of said main burner for controlling the conductivity of saidelectron discharge device with variations in said conditions, and anelectric igniter forlsaid pilot burner connected across said source inparallel circuit with said resistor means and being controlled byoperation of said armature means.

13. A temperature and safety control system for fuel burning apparatushaving main and pilot burners and asource of electric energy, comprisingelectromagnetic valve means for controlling the supply of fuel to themain burner, a control relay having a coil and armature means operablethereby, a timing device having contacts controlling the energization ofsaid coil ,for a predetermined period during which said coil iseffectivelydeenergized, -means operable in one position of said armaturemeans-for establishing a holding circuit for said coil independent ofsaid timing device, an electron discharge device having anode andcathode elements and a pair of gridsyconnections between said anode andcathode elements and said electromagnetic valve means controlled byoperation of said armature means, potential divider means connectedacross said source and tapped to said cathode for maintaining asubstantially fixed potential, a pair of resistors in parallel circuitwithsaid potential divider means, one said resistor being exposed to theflame of the pilot burner for varying in resistance value when heatedthereby, said pair of resistors having common connection with one ofsaid grids, thermally responsive means connected to said source and theotherof said grids and located to respond to temperature conditionscaused by operation of said main burner for applying out-of-phasevoltage to-said other grid upon an increase in said temperature andvarying the conductivity of said electron discharge device while saidone resistor remains heated, and an electric igniter for said pilotburner connected across said source in parallel circuit with said pairof resistors and being controlled by operation of said armature means.

ROBERT CQ'MAIN.

REFERENCES CITED The following references are of record in the file ofthis patent:

I UNITED STATES PATENTS Number 2,404,643 Livingston July 23, 1946

